Semiconductor materials such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN) are used in many different semiconductor devices. In particular, such semiconductor materials are used in transistors, and in vertical-cavity surface-emitting lasers (VCSELs), in-plane lasers, and light-emitting diodes (LEDs structured to emit light ranging in wavelength from red through ultra-violet. In the following description, the term gallium nitride material will be understood to refer to any of the above-mentioned semiconductor materials.
Semiconductor devices made from gallium nitride materials commonly include one or more electrical contacts through which electric current received via a bonding wire is distributed across the surface of the gallium nitride material for conduction through the bulk of the gallium nitride material. To minimize beat generation in the semiconductor device, the electrical resistance of the electrical contact, and the voltage drop across the p-contact, should be a minimum.
Electrical contacts made to gallium nitride materials include n-contacts made to gallium nitride materials doped with donor impurities, and p-contacts made to gallium nitride materials doped with acceptor impurities. Of these two types of contact, obtaining a p-contact with a low contact resistance and low voltage drop is the more difficult.
Conventional semiconductor devices using gallium nitride materials form a p-contact by depositing a conductive metal directly on the surface of the p-type gallium nitride material. For example, in such conventional devices, a thin layer of a metal such as titanium, nickel or palladium may be deposited directly on the surface of the p-type gallium nitride material. A much thicker layer of gold is then deposited on the thin layer of metal. A thin intermediate layer of platinum may be interposed between the thin layer of metal and the thicker layer of gold.
Two main mechanisms prevent a conventional p-contact from having the desirable electrical characteristics stated above. First, the large work function difference between the gallium nitride material and the metal establishes a high potential barrier between the gallium nitride material and the metal. For gallium nitride, the potential barrier is typically about 3.4 V. Second, the maximum level of activated acceptor impurities that currently can be reliably achieved in gallium nitride materials is between 10.sup.17 -10.sup.18 atoms.cm.sup.-3. This results in the p-type gallium nitride having a contact resistivity in the range 10.sup.-1 -10.sup.-2 ohm.cm.sup.2. This maximum level of activated acceptor impurities is at least one order of magnitude below that which will provide a contact resistivity in the range 10.sup.-4 -10.sup.-5 ohm.cm.sup.2. A contact resistivity in the range 10.sup.-4 -10.sup.-5 ohm.cm.sup.2 is desirable for the p-contact to have an acceptably-low electrical resistance.
The low concentration of activated acceptor (p-type) impurities that can be achieved in a gallium nitride material is especially problematical in combination with the high potential barrier between the gallium nitride material and the layer of metal because it results in a wide depletion region that extends into the gallium nitride material from the interface between the layer of metal and the gallium nitride material.
Moreover, in practical devices, the depletion zone is wider than that predicted from the potential barrier and the level of activated acceptor impurities in the gallium nitride material. This is because the effective level of activated acceptor impurities in the gallium nitride material next to the metal contact is lower than in similarly-doped bulk material. At the metal-gallium nitride material interface of the p-contact, the metal typically reacts with the nitrogen of the gallium nitride material to form metal nitrides. This removes nitrogen atoms from the gallium nitride material, leaving nitrogen vacancies in the gallium nitride material. The nitrogen vacancies act as donor sites that neutralize adjacent acceptor sites in the gallium nitride material and lower the effective concentration of activated acceptor impurities. This further widens the depletion zone.
The voltage drop across conventional light-emitting devices using gallium nitride material is typically in the range of 5-7 volts. Of this voltage drop, only about 3 volts is accounted for by the diode voltage of the device. Most of the rest of the voltage drop is due to the voltage drop of the p-contact.
A p-contact for a gallium nitride material that does not suffer from the disadvantages of known p-contacts is desirable, since such a p-contact would reduce heat dissipation in the semiconductor devices employing such contacts. In particular, a p-contact that would enable a gallium nitride material semiconductor device, such as a light-emitting device, to have a forward voltage drop close to the diode voltage of the gallium nitride material is desirable.